The present invention generally relates to an anisotropic conductive adhesive used to anisotropic conductively connect an LED element to an electrode substrate, and an LED light emitting device that has an LED element mounted on an electrode substrate using the anisotropic conductive adhesive.
Optical function elements that use an LED are attracting attention in recent years.
Optical function elements of this type employ flip chip mounting in which an LED chip is directly mounted on a wiring board, for the purpose of size reduction or the like.
Various methods of mounting an LED chip on a wiring board by flip chip mounting have hitherto been known as shown in FIG. 7(a) to FIG. 7(c).
FIG. 7(a) shows a mounting method that uses wire bonding. In a light emitting device 101 of FIG. 7(a), a die bonding adhesive 110 and a die bonding adhesive 111 fix an LED chip 103 onto a wiring board 102 in a state such that a first electrode 104 and a second electrode 105 of the LED chip 103 are positioned upward side (the side opposite from the wiring board 102).
By bonding wires 106 and 108, a first pattern electrode 107 and a second pattern electrode 109 on the wiring board 102 are electrically connected to the first electrode 104 and the second electrode 105 of the LED chip 103, respectively.
FIG. 7(b) shows a mounting method that uses conductive paste.
In a light emitting device 121 shown in FIG. 7(b), a first electrode 104 and a second electrode 105 of an LED chip 103 are electrically connected to a first pattern electrode 124 and a second pattern electrode 125 of a wiring board 102 by conductive paste 122 and conductive paste 123, which are, for example, copper paste in a state such that the first electrode 104 and the second electrode 105 of the LED chip 103 face toward the wiring board 102. The LED chip 103 is also adhered onto the wiring board 102 by a sealing resin 126 and a sealing resin 127.
FIG. 7(c) shows a mounting method using an anisotropic conductive adhesive.
In a light emitting device 131 shown in FIG. 7(c), a first electrode 104 and a second electrode 105 of an LED chip 103 are electrically connected to a bump 132 and a bump 133, which are formed on a first pattern electrode 124 and a second pattern electrode 125 of a wiring board 102, respectively, by conductive particles 135 in an anisotropic conductive adhesive 134 in a state such that the first electrode 104 and the second electrode 105 of the LED chip 103 face toward the wiring board 102. The LED chip 103 is also adhered onto the wiring board 102 by an insulating adhesive resin 136 in the anisotropic conductive adhesive 134.
However, the conventional art described above has various problems.
Firstly, in the mounting method using wire bonding, light emission efficiency is low because the bonding wires 106 and 108, which are made of gold, absorb light having a wavelength of, for example, 400 nm to 500 nm.
In this method, cure time of the die bonding adhesive 110 and the die bonding adhesive 111 is long because of the use of an oven for the curing, and it is difficult to improve production efficiency.
In the mounting method using the conductive paste 122 and the conductive paste 123, on the other hand, the adhesion force of the conductive paste 122 and the conductive paste 123 alone is weak and needs additional strength by the sealing resin 126 and the sealing resin 127. The sealing resin 126 and the sealing resin 127, however, degrade light emission efficiency by causing light diffusion or light absorption into the conductive paste 122 and the conductive paste 123.
This method also suffers from a long cure time for the sealing resin 126 and the sealing resin 127 because of the use of an oven for the curing, which makes it difficult to improve production efficiency.
In other mounting methods where the anisotropic conductive adhesive 134 is used, the color of the conductive particles 135 in the anisotropic conductive adhesive 134 is brown so that the insulating adhesive resin 136 is consequently colored brown as well. The coloring causes the anisotropic conductive adhesive 134 to absorb light, which thereby degrades light emission efficiency.
As a solution to those problems, an anisotropic conductive adhesive has been proposed that does not degrade light emission efficiency by forming a conductive layer using silver (Ag) having high light reflectance and low electric resistance, and thus reducing the absorption of light.
However, silver is a chemically unstable material and has a problem of being susceptible to oxidation and sulfuration. Another problem with using silver is that energization after thermal compression bonding causes migration, which leads to a breakage in wiring and to decrease in adhesion strength due to the deterioration of the adhesive.
In order to solve the above-discussed problem, an Ag-based alloy thin film that has excellent reflectance, corrosion resistance, and anti-migration properties has been proposed, for example, see JPA2008-266671, JPA 2005-120375, JPA H05-152464 and JPA 2003-26763.
Covering the surface of a conductive particle with this Ag-based alloy thin film improves corrosion resistance and anti-migration properties. However, the use of, for example, nickel in a base layer in combination with the Ag-based alloy thin film as the topmost layer gives rise to a problem such that the entire reflectance of the conductive particles is degraded by the reflectance of nickel, which is lower than the reflectance of Ag.
In addition, Au or Ni exposed on the surface of the conductive particles 135 causes light absorption and consequently decrease light emission efficiency.